His strength-boosting ability goes well with anything, especially with a few heavy-hitters such as the aforementioned Toadstool, as well as Smoosh-Shroom. However, it does interact negatively when you want to play Doom-Shroom.

You can also use Rescue Radish to Bounce him, so he can be played again and Conjure another mushroom. However, only Nightcap can do this normally.

Against

Cosmic Mushroom himself isn't very much of a threat, but you should be prepared for any mushroom he Conjures, as your opponent could surprise you with something like Sportacus or Smoosh-Shroom.

Gallery

Cosmic Mushroom's statistics

Cosmic Mushroom's statistics after being Conjured by another Cosmic Mushroom